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SWTPC SWTBUG® (SWATBUG) MONITOR ROM
One of the features of the SWTPC 6800 Computer System is that the conventional
programmer’s console has been replaced with a monitor ROM. The programmer’s console
consists of all the pretty switches and lights often found on similar microcomputers that are used
to bootstrap the system after power up. The programmer’s console not only raises the cost of the
system, but more often than not is confusing and tedious to use for both be-ginning and
experienced programmers. The monitor ROM on the other hand is a permanently stored program
that gives the computer the intelligence required to communicate with the operator thru an
interfaced terminal system immediately after power up without flipping switches for 10 minutes.
This technique makes the computer do the work of simplifying communication between itself and
the operator.
SWTBUG® is the name of the monitor program used in the SWTPC 6800 Computer
System. It might be thought of as kind of a mini-operating system since it gives the operator
command control over the computer system.
Features of the SWTBUG® ROM include:
∗ Memory Examine and Change
∗ Program loading from cassette or paper tape thru the control interface or thru I/O port # 0.
∗ Program saving to cassette or paper tape
∗ Go to user program
∗ Display contents of registers
∗ Erase SWTPC CT-1024 terminal system screen
∗ SWTPC MF-68 floppy disk boot
∗ Byte search
∗ Breakpoint debugging
∗ Vectored hardware and software interrupts to user defined addresses
SWTBUG® is a permanently stored program and cannot be erased or lost by either a loss of
power or user program error. It is always resident in the computer while power is ON and need
never be loaded into the machine. Subroutines within the ROM are documented and available to
the user to simplify programming and conserve on the use of user RAM memory. Character input
and output, string output and return to monitor are just a few subroutines available to the user.
SWTBUG® is a 1K byte program and is addressed high in memory, far above the amount of
RAM memory required for most user programs. Since SWTBUG® does require a small amount of
RAM memory for operation, a 128 byte scratchpad RAM has been implemented on the processor
board so that no external user RAM memory is required for monitor operation. There is even
enough extra room in this RAM so that short programs such as memory diagnostics can be
loaded into and run from the scratchpad RAM without requiring any external user RAM memory.
Extra care however must be exercised to avoid overstoring any memory locations required for
proper monitor operation. Complete de-tails on this are given later in this writeup. The
SWTBUG® ROM is located from memory addresses E000 thru E3FF. The scratchpad RAM is
located from memory addresses A000 thru A07F. Both components are physically located on the
processor board and are functional any time the system is powered up. Whenever computer
control is transferred from SWTBUG® to the user program, there are only four ways to get back
into the SWTBUG® command mode. The first is to put a jump to the CONTRL entry point
address within SWTBUG® as the last step of your program. The second is to incorporate a
command or action within your program which transfers program control to the CONTRL entry
point address within SWTBUG® The third is to depress the front panel RESET button. The fourth
is turn the computer OFF and then back ON again. The fourth method is rather drastic and wipes
out all RAM memory data, it is only mentioned to let you know that the computer always powers
up with the SWTBUG® monitor in the command mode.